1. Field of the Invention
The present invention relates to a passive safety device for the protection of the occupants of a vehicle, and more specifically to supplying power to a passive safety device when a main power source is interrupted.
2. Description of the Related Art
A conventional passive safety device for vehicles such as, for example, an airbag system, is described in JP-A-2004-255911, which includes a switching element for safing, a HI-side switching element, a LO-side switching element, and a drive circuit for driving the switching elements. The switching element for safing, HI-side switching element, and LO-side switching element are connected in series. The switching element for safing is connected to a power source. A squib is placed between the HI-side switching element and the LO-side switching element.
In a typical scenario, the power source is constructed of a battery, a voltage converter, and a backup power source. The voltage converter increases the output voltage of the battery and outputs it. The backup power source is charged with the output voltage of the voltage converter. When the output of the voltage converter is interrupted, the backup power source supplies voltage in place of the voltage converter. The switching element for safing is driven by the drive circuit and turns the output voltage of the voltage converter or the backup power source to a predetermined target voltage and supplies it to the Hi-side switching element.
During backup operation, the output voltage of the backup power source drops with the lapse of time. When the switching element for safing is a FET (Field-Effect Transistor), for example, the following takes place: when the output voltage of the backup power source becomes less than or equal to a target voltage, the output source voltage cannot be controlled to the target voltage. It is therefore required to fully turn on the switching element for safing to directly supply the output voltage of the backup power source to the HI-side switching element. To fully turn on the switching element for safing, the gate voltage must be made higher than the source voltage. Using only the backup power source, however, the gate voltage cannot be made higher than the source voltage. Consequently, a charge pump circuit is generally used as a power source for the drive circuit so that a certain gate voltage can be ensured even when the output voltage of the backup power source drops during backup operation.
However, a charge pump circuit is complicated and requires an increased number of parts. Therefore, use of a charge pump circuit makes it difficult to reduce the cost of an airbag system.